Electrical explosion forming



1965 E. M. CALLENDER 3,200,626

ELECTRICAL EXPLOSION FORMING Filed Dec. 26, 1961 2 Sheets-Sheet 1 i Fig. 3

INVENTOR.

EDWIN M. CAL LENDER W h AL E JEIM V.

Aug. 17, 1965 E. M- CALLENDER ELECTRICAL EXPLOSION FORMING 2 Sheets-Sheet 2 Filed Dec. 26, 1961 INVENTOR EDWIN M. CALLENDER ATTORNEY United States Patent 3,200,626 ELEGTRICAL EXPLGSKUN FORMENG Edwin Meyers Callendcr, Bala-Cynwyd, Pa assignor to General Electric Company, a corporation of New York Filed Dec. 26, 196i, Ser. No. 161,819 9 Claims. (Ci. 72-456) This invention relates generally to electrical explosion forming and more particularly to improved apparatus for producing pressure waves by electrical explosions.

It is known that an explosion in a liquid produces shock waves in the liquid. If these shock waves are of sufficient intensity they can be used to force a sheet or blank against a die and so form the blank to a desired configuration.

Chemical explosives have been used to produce the required explosions; however, the hazardous nature of such explosives prevents their use in certain zoned areas. More recently, an electrical discharge in a liquid, which may be called an electrical explosion, has been used to produce the shock waves. Such an electrical explosion can be produced by providing spaced electrodes in the liquid and striking an are between them.

Where the liquid utilized to transfer the shock waves is water, a potential difference of approximately 100,000 volts per inch of electrode separation is required before the arc will strike. Voltages of this magnitude become prohibitively expensive to produce, consequently the electrodes are normally closely spaced and the shock waves produced by the electrical explosion effectively emanate from a point source.

Shock waves emanating from a point source are adequate for certain types of explosion forming, for example when the blank is small, but they are not the ideal waves when a large plate or sheet is to be formed. For large sheets a wave having a broad front such as would be obtained from a linear or planar source is desired.

One method of achieving a shock wave having a broader front, which has been utilized in the past, is to connect a wire between the electrodes. Such a wire serves to reduce the potential difference needed to cause an electrical discharge while also directing the arc in a desired path. To the observer it appears that certain wires explode under the influence of the arc. What actually occurs is an almost instantaneous melting and vaporization of the wire material and this suddenly increased volume of the wire due to its vaporization causes shock waves in the hydraulic fluid which can be used for explosion forming.

The intensity of the shock waves produced by this method is increased as a greater quantity of wire is vaporized, due to the greater volume of liquid which is displaced. However, the usefulness of the v-aporizable wire has been limited, since it has been heretofore possible to explode only fine wires and the resulting shock waves do not have the capacity to form thick sheets or blanks.

It is therefore an object of this invention to provide electrical explosion forming apparatus which will produce high intensity shock waves from a linear source.

It is also an object of this invent-ion to provide electrical explosion forming apparatus which will produce shock waves from a planar source.

In a preferred form of the invention, a heat insulating path or passageway with a material capable of supporting an electrical discharge contained within it, is provided through the liquid to extend between the electrodes of the explosion forming apparatus. This passageway serves the function of restricting the transfer of heat from the material contained within it to the liquid, thereby permitting the material to reach its vaporization tem- "ice perature. The passageway is designed to fracture when the electrical explosion occurs; hence it does not impede the production of shock waves.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a schematic cross-sectional side elevation of a typical electrical explosion forming apparatus;

FIGURE 2 is a schematic, cross-sectional front elevation of a first embodiment of the invention;

FIGURE 3 is a schematic, cross-sectional front elevation of a second embodiment of the invention;

FIGURE 4 is a schematic, cross-sectional side elevation of electrical explosion forming apparatus employing the invention;

FIGURE 5 is a schematic, isometric view, partially cut away, of another embodiment of the invention; and

FIGURE 6 is a schematic, cross-sectional front elevation of another embodiment of the invention.

Referring to FIGURE 1, a tank or chamber 20 is shown containing a liquid 22 such as water. Positioned near one end of chamber 20 is a die 24 having a working surface 26 against which a sheet or blank 28 is to be pressed. As with any die, working surface 25 is designed to yield a desired configuration to blank 28 when the blank is forced against it. Die 24 may be provided with one or more ports 30 so that the space between working surface 26 and blank 28 can be evacuated by means of exhaust tube 32.

The desired degree of vacuum can best be achieved if a suitable seal such as G-ring 34 is provided between blank 28 and die 24. In addition some .type of fastening or securing means, such as clamp 3% held by bolts 37 may be provided to support blank 2% adjacent to die 24.

Shock wave producing apparatus, not shown in FIG- URE l, is positioned in chamber 2t? so that it is capable of producing an electrical explosion or are 36 which is directed in a line substantially parallel to blank 28. An are such as this will produce shock waves which will emanate radially, as indicated by the arrows, with only a small portion of the waves produced impinging directly upon blank 28. A more effective utilization of the energy available can be achieved by providing a reflector 38 having a reflecting surface 4-0 which may be of parabolic cross-section. If are 36 is then positioned at the focus of the parabola, the maximum use of the energy available will result and a shock wave approaching a planar form can be achieved. Because the pressures involved are of high magnitude, it may be desirable to make reflector 33 a massive structure, although it may be possible to make reflector 3% an integral part of chamber 20.

Referring next to FIGURE 2, one embodiment of a shock wave producing apparatus will now be described. Electrodes 44 are mounted in the walls of chamber 20 with suitable insulators 46. In accordance with the invention, enclosing the tips of electrodes 44, and extending between them, a heat insulating passageway 43 is provided. Passageway 43 may be a glass or plastic tube and is preferably relatively thin and easily fractured. Suitable plastics for passageway 48 are plexiglass, or polyvinyl or polyethyl resins. In order to produce a shock wave of sufficient magnitude, a material such as wire 5'0, capable of supporting an electrical discharge is provided to ,be contained in passageway 48. Wire 50 is connected between electrodes 44 and may be of copper, nickel, etc.

When electrodes 44 are connected between the terminals of a suitable power supply, such as a bank of capacitors, an electrical discharge will occur. The are current will initially be carried by wire 50, but the high current will rapidly melt and then boil or vaporize the wire. Passageway 48 prevents the surrounding liquid 22 from contacting wire 50 thereby reducing heat transfer from the wire by conduction. This heat insulation is needed because the temperatures of vaporization is so high (2300 C. for copper for example), that the heat generated by the discharge current would be transferred to the liquid at such a rapid rate that the required temperature of vaporization might never be reached.

In the same manner that the boiling of water can be used to generate high steam pressures, the vaporization of the wire results in the generation of high pressures which break passageway 48 and displace a quantity of liquid 22. Since the vaporization occurs almost instantly, the sudden displacement of liquid 22 produces a shock wave in this liquid along the length of passageway 4-8. This shock wave is transferred by liquid 22 until it strikes blank 28,-forming this blank to the configuration of the working surface 26 of the die 24.

In addition to restricting the transfer of heat by conduction to liquid 22, passageway 48 can retard the-radiation of heat from wire 50 to liquid 22. As shown in FIGURE 2, a heat reflective coating 52, such as chro rniurn, may be applied to the inner surface of passageway 4-8. If passageway 43 were made of a transparent material .t-he coating could be .applied to its outer surface instead.

Although wire St) is shown in FIGURE 2 as being spaced from passageway 48, it is possible for some applications to let wire 50 completely fill passageway 48. This modification can be easily constructed by coating wire t} with a plastic, such as Krylon, or with enamel instead of providing a separate tube. Thus in FIGURE 6, in which the same identifying numbers are used as were used to identify similar parts in FIGURE 2, wire 50 and electrodes 44 are provided. with insulating coating 70 sealing them from the fluid.

The shock waves which are generated when a wire is utilized to produce the electrical explosion are of high intensity; but relatively short duration since the arc is rapidly quenched. For some purposes it may be preferable to extend the duration of the shock wave. A vaporizable salt, such as potassium hydroxide, can be used to line passageway 48 or coat wire 50. Such a salt has a tendency to sustain the are for a longer time while at the same time it is the source of'quantities of gas which lend additional force to the shock wave.

As was mentioned previously, electrodes 44 may be connected between the terminals of a suitable power supply when it is desired to strike an arc. Because the voltages involved are relatively high, it may be preferred to perform the necessary switching within the shock producing apparatus and thereby eliminate the need for external switch-gear. An embodiment of the shock producing apparatus having switching capability will now be described with reference to FIGURE 3. The same identifying numbers are used in FIGURE 3 as were used to identify similar parts in FIGURE 2.

Electrodes 44 are again maintained in the walls of chamber by means of insulators d6. Passageway 54 is provided .to enclose and extend between electrodes 44. Initially contained within passageway 54 is an electrically insulating fluid such as oil. By this arrangement electrodes 54 can be connected to the capacitor blank before a charge is built up in the capacitors without an are being struck after the charge is accumulated. When electrical explosion forming is to occur, a purging material such .as helium or argon is supplied to passageway 54 through inlet 56, thereby discharging the insulating fluid through outlet 53 and causing the discharge to result.

The various embodiments of the shock wave producing apparatus whichhave been described provide a linear source of shock waves. Such a linear source can be located at the axis of a cylindrical body which is to be formed, and also provides a convenient solution where a Wide. shock wave is needed. By the use of a parabolic 4 reflector as described with reference to FIGURE 1, a linear source can also yield shock waves of various heights. The energy available from a particular linear source is substantially constant; however, so as the wave is made higher, the energy per square inch of the wave front decreases. 7

Referring to FIGURE 4, an alternative method of generating a planar shock wave to achieve high energy potentials will'be described. The arrangement ofchamber 20, die 24, and blank 28 may remain substantially as described in relation to FIGURE 1. Instead of a single are such as might be used with the apparatus of FIGURE 1, a plurality of arc paths 69 are provided in what may be a parallel arrangement. The reflector 38 would in this case not utilize a parabolic reflecting surface but may be designed so as to have a planar reflecting surface. The individual .arcs 60 can be directed'in the manner heretofore described with a pair of electrodes (not shown) being provided for each of the arcs.

Where it is desired to have a wave front of some particular shape other than planar, the multiple arcs can be arranged so as to have the desired wave front profile. The portions of the wave resulting from the arcs closest to the blank will then strike the blank first.

It is also possible to utilize a single arc path to achievea planar wave front configuration by providing an arc passageway which is planar in design. Such a planar arc passageway may be constructed as shown in FIGURE 5. An electrically conducting sheet or ribbon 64 of metal such as aluminum foil is connected between electrodes 66. In order to insulate this sheet of foil from the fluid content in chamber 20, coating 68 of a. frangible insulating material such as plastic or enamel is applied to sheet 64. This coating maybe extended to also cover electrodes 66 as shown, or separate insulation may be provided for this purpose.

For some purposes it may be desirable to use an electrically conducting liquid such as mercury as the hydraulic liquid for the electrical explosion forming. The invention has added utility here since an arc struck in such a medium may have an erratic path. It is necessary in such an application that the passageway or means containing the material to support the discharge be electrically insulating as well as heat insulating By utilizing the shock producing apparatus of this invent-ion it becomes practical to form, by electrical explosion forming, metal sheets on the order of twice the thickness possible with the prior arrangements.

Although the invention has been described with reference to utilization in apparatus designed to form flat sheets, it should be understood it would be equally applicable to the forming of other shaped blanks, e.g., cylindrical. Changes such as this can'be made without departing from the spirit of the invention and the scope of the appended claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. Apparatus for electrical explosion forming comprising:

a chamber containing a fluid,

a die having a working surface positioned in said chamber,

a means forsupporting ing surface,

a pair of spaced electrodes positioned in said'chamber,

containing means for containing amaterial capable of supporting an electrical discharge extending between said electrodes and through said fluid and sealing said material from said fluid, and a material capable of supporting an electrical discharge contained within said containing means. t

2. In apparatus for electrical explosion forming hava chamber containing a fluid,

a blank adjacent to said worka die having a working surface positioned in said chamber, and means for supporting a blank adjacent to said Working surface, the improvement comprising:

a pair of spaced electrodes positioned in said chamber, a tube enclosing and extending between said electrodes and through said fluid, and a material capable of supporting an electrical discharge contained in said tube. 3. Apparatus for electrical explosion forming comprising:

a chamber containing a fluid, a die having a working surface positioned in said chamber, means for supporting a blank adjacent to said working surface, a pair of spaced electrodes positioned in said chamber, a heat insulating tube enclosing and extending between said electrodes and through said fluid, and a materialcapable of supporting an electrical discharge contained in said tube. 4. In apparatus for electrical explosion forming having:

a chamber containing a fluid, a die having a working surface positioned in said chamber, and means for supporting a blank adjacent to said working surface, the improvement comprising:

a pair of spaced electrodes positioned in said chamber, a glass tube enclosing and extending between said electrodes and through said fluid, said glass tube having a heat reflective coating,

and a metal wire connected between said electrodes and contained within said glass tube. 5. Apparatus for electrical explosion forming comprising:

a chamber containing a hydraulic fluid, a die having a working surface positioned in said chamber, means for supporting a blank adjacent to said working surface, a pair of spaced electrodes positioned in said chamber, a tube containing an electrically insulating fluid enclosing and extending between said electrodes and through said hydraulic fluid, and means for purging said tube of said insulating fluid with a material capable of supporting an electrical discharge. 6. Apparatus for electrical explosion forming comprising:

a chamber containing a fluid, a die having a working surface positioned in said chamber,

means for supporting a blank adjacent to said working surface, a pair of spaced electrodes positioned in said chamber, an electrically conducting wire connected between said electrodes, and an insulating coating sealing said wire and said electrodes from said fluid. 7. Apparatus for electrical explosion forming comprising:

a chamber containing a fluid, a die having a working surface positioned in said chamber, means for supporting a blank adjacent to said working surface, a plurality of pairs of spaced electrodes positioned in said chamber, said pairs of electrodes so arranged that the imaginary surface formed by connecting them is substantially equivalent to a desired wave front, and means for containing a material capable of supporting an electrical discharge extending between each of said pairs of electrodes and through said fluid. 8. Apparatus for electrical explosion forming comprising:

a chamber containing a fluid, a die having a working surface positioned in said chamber, means for supporting a blank adjacent to said working surface, a pair of spaced electrodes positioned in said chamber, a flat conducting sheet connected between said electrodes, and an insulating coating shielding said sheet and said electrodes from said fluid. 9. Apparatus for electrical explosion forming comprising:

a chamber containing an electrically conducting fluid, a die having a working surface positioned in said chamber, means for supporting a blank adjacent to said working surface, a pair of spaced electrodes positioned in said chamber, a heat and electrically insulating tube enclosing and extending between said electrodes and through said fluid, and a material capable of supporting an electrical discharge contained in said tube.

References Cited by the Examiner FOREIGN PATENTS 3/58 Russia.

OTHER REFERENCES CHARLES W. LANHAM, Primary Examiner.

WILLIAM J. STEPHENSON, Examiner. 

1. APPARATUS FOR ELECTRICAL EXPLOSION FORMING COMPRISING: A CHAMBER CONTAINING A FLUID, A DIE HAVING A WORKING SURFACE POSITIONED IN SAID CHAMBER, A MEANS FOR SUPPORTING A BLANK ADJACENT TO SAID WORKING SURFACE, A PAIR OF SPACED ELECTRODES POSITIOED IN SAID CHAMBER, CONTAINING MEANS FOR CONTAINING A MATERIAL CAPABLE OF SUPPORTING AN ELECTRICAL DISCHARGE EXTENDING BETWEEN SAID ELECTRODES AND THROUGH SAID FLUID AND SEALING SAID MATERIAL FROM SAID FLUID, AND A MATERIAL CAPABLE OF SUPPORTING AN ELECTRICAL DISCHARGE CONTAINED WITHIN SAID CONTAINING MEANS. 